Spatial puncturing apparatus, method, and system

ABSTRACT

Stations in an N×N multiple-input-multiple-output (MIMO) wireless network always puncture the weakest spatial channel. A receiving station determines channel state information for N spatial channels and feeds back to the transmitting station channel state information for only N−1 spatial channels. The channel state information may include a beamforming matrix to cause the transmitting station to utilize N−1 spatial channels.

FIELD

The present invention relates generally to wireless networks, and morespecifically to wireless networks that utilize multiple spatialchannels.

BACKGROUND

Closed loop multiple-input-multiple-output (MIMO) systems typicallytransmit channel state information from a receiver to a transmitter.Transmitting the channel state information consumes bandwidth that wouldotherwise be available for data traffic.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram of two wireless stations;

FIG. 2 shows a flowchart in accordance with various embodiments of thepresent invention;

FIG. 3 shows simulation results;

FIG. 4 shows a diagram of a wireless communications device;

FIG. 5 shows dimensions of a channel state information matrix;

FIG. 6 shows a diagram of a wireless communications device; and

FIG. 7 shows a system diagram in accordance with various embodiments ofthe present invention.

DESCRIPTION OF EMBODIMENTS

In the following detailed description, reference is made to theaccompanying drawings that show, by way of illustration, specificembodiments in which the invention may be practiced. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the invention. It is to be understood that the variousembodiments of the invention, although different, are not necessarilymutually exclusive. For example, a particular feature, structure, orcharacteristic described herein in connection with one embodiment may beimplemented within other embodiments without departing from the spiritand scope of the invention. In addition, it is to be understood that thelocation or arrangement of individual elements within each disclosedembodiment may be modified without departing from the spirit and scopeof the invention. The following detailed description is, therefore, notto be taken in a limiting sense, and the scope of the present inventionis defined only by the appended claims, appropriately interpreted, alongwith the full range of equivalents to which the claims are entitled. Inthe drawings, like numerals refer to the same or similar functionalitythroughout the several views.

FIG. 1 shows a diagram of two wireless stations: station 102, andstation 104. In some embodiments, stations 102 and 104 are part of awireless local area network (WLAN). For example, one or more of stations102 and 104 may be an access point in a WLAN. Also for example, one ormore of stations 102 and 104 may be a mobile station such as a laptopcomputer, personal digital assistant (PDA), or the like.

In some embodiments, stations 102 and 104 may operate partially incompliance with, or completely in compliance with, a wireless networkstandard. For example, stations 102 and 104 may operate partially incompliance with a standard such as ANSI/IEEE Std. 802.11, 1999 Edition,although this is not a limitation of the present invention. As usedherein, the term “802.11” refers to any past, present, or future IEEE802.11 standard, including, but not limited to, the 1999 edition.

Stations 102 and 104 each include multiple antennas. Station 102includes “N” antennas, and station 104 includes “M” antennas, where Nand M may be any number. Further, N and M may or may not be equal. Theremainder of this description discusses the case where N and M areequal, but the various embodiments of the invention are not so limited.The “channel” through which stations 102 and 104 communicate may includemany possible signal paths. For example, when stations 102 and 104 arein an environment with many “reflectors” (e.g. walls, doors, or otherobstructions), many signals may arrive from different paths. Thiscondition is known as “multipath.” In some embodiments, stations 102 and104 utilize multiple antennas to take advantage of the multipath and toincrease the communications bandwidth. For example, in some embodiments,stations 102 and 104 may communicate usingMultiple-Input-Multiple-Output (MIMO) techniques. In general, MIMOsystems offer higher capacities by utilizing multiple spatial channelsmade possible by multipath.

In some embodiments, stations 102 and 104 may communicate usingorthogonal frequency division multiplexing (OFDM) in each spatialchannel. Multipath may introduce frequency selective fading which maycause impairments like inter-symbol interference (ISI). OFDM iseffective at combating frequency selective fading in part because OFDMbreaks each spatial channel into small subchannels such that eachsubchannel exhibits a more flat channel characteristic. Scalingappropriate for each subchannel may be implemented to correct anyattenuation caused by the subchannel. Further, the data carryingcapacity of each subchannel may be controlled dynamically depending onthe fading characteristics of the subchannel.

MIMO systems may operate either “open loop” or “closed loop.” In openloop MIMO systems, a station estimates the state of the channel withoutreceiving channel state information directly from another station. Ingeneral, open loop systems employ exponential decoding complexity toestimate the channel. In closed loop systems, communications bandwidthis utilized to transmit current channel state information betweenstations, thereby reducing the necessary decoding complexity, and alsoreducing overall throughput. The communications bandwidth used for thispurpose is referred to herein as “feedback bandwidth.” When feedbackbandwidth is reduced in closed loop MIMO systems, more bandwidth isavailable for data communications.

Three types of receiver architectures for MIMO systems include: linear,iterative, and maximum-likelihood (ML). In open-loop operation, MLreceivers have much better performance than linear and iterativereceivers. For example, at 1% packet error rate and 4×36 Mbps, MLreceivers are 12 dB more power efficient than linear and iterativereceivers, or equivalently, have four times better propagation range.However, ML receivers need 2×10⁵ times more multiplication operationsthan linear and iterative receivers. To approach the performance of MLreceivers with the complexity of linear receivers, and to reduce thefeedback bandwidth, the various embodiments of the present inventionutilize deterministic spatial channel puncturing with closed-loopoperation.

As used herein, “puncturing” refers to the non-use of a particularspatial channel. For example, in a N×N MIMO system, various embodimentsof the present invention use N−1 channels instead of N channelsregardless of the instantaneous channel state information. The spatialpuncturing is deterministic in the sense that one spatial channel isalways punctured, and an N×N system will always use N−1 spatialchannels. By always only utilizing N−1 spatial channels in a N×N MIMOsystem, the amount of channel state information to be transmitted isreduced, and the feedback bandwidth is reduced.

FIG. 2 shows a flowchart in accordance with various embodiments of thepresent invention. In some embodiments, method 200 may be used in awireless system that utilizes MIMO technology. In some embodiments,method 200, or portions thereof, is performed by a processor orelectronic system, embodiments of which are shown in the variousfigures. In other embodiments, method 200 is performed by a wirelesscommunications device. Method 200 is not limited by the particular typeof apparatus or software element performing the method. The variousactions in method 200 may be performed in the order presented, or may beperformed in a different order. Further, in some embodiments, someactions listed in FIG. 2 are omitted from method 200.

Method 200 is shown beginning at block 210 in which a receiving stationreceives a training pattern from a transmitting station. For example,station 102 may transmit a training pattern, and station 104 may receivethe training pattern. At 220, the receiving station estimates N spatialchannels, where N is equal to a number of receiving antennas. In someembodiments, this may correspond to station 104 computing a currentchannel matrix describing the current state of the N spatial channels.At 230, the receiving station determines the weakest of the N spatialchannels, and at 240, the receiving stations transmits back the channelstate information describing the N−1 spatial channels. In someembodiments, the channel state information is in the form of a transmitbeamforming matrix. In these embodiments, the receiver computes atransmit beamforming matrix from the current channel matrix and thensends the beamforming matrix back to the transmitter. In variousembodiments of the present invention, one spatial channel is alwayspunctured, and the transmit beamforming matrix is reduced in size,thereby reducing the feedback bandwidth. Mathematical descriptions ofvarious acts shown in FIG. 2 are provided below.

Let the input/output (I/O) model bey=Hx+z

where x_(i) is the signal on the ith transmit antenna, y_(i) is thesignal received at the ith receive antenna, H_(ij) is the channel gainfrom the jth transmit antenna to the ith receive antenna, and z_(i) isthe noise on the ith receive antenna. In closed-loop MIMO, the receivermay send a pre-coding matrix P back to the transmitter and the I/O modelbecomesy=HPx+z

Upon singular value decomposition (SVD), we haveH=UΣV ^(y)

where U and V are N×N unitary matrices, and Σ is a diagonal matrix withpositive entries. Matrix V is the transmit beamforming matrix. When Vrepresents N spatial channels, V includes 2N² real numbers, and when Vrepresents N−1 channels, V includes 2N(N−1) real numbers.

Assume elements of H are independent complex Gaussian distributed withzero mean and unit variance. Denote the gain of the ith spatial channelas λ_(i)(λ₁≧λ₁≧ . . . ≧λ_(N)), where λ_(i) denotes the entries indiagonal matrix Σ. The distribution of λ_(N) can be shown asf(λ)=Ne ^(−λN),from which its expected value may be derived as

${E\left\lbrack \lambda_{N} \right\rbrack} = {\frac{1}{N}.}$

Also, the overall expected value for λ_(i) may be derived as

${E\left\lbrack {\frac{1}{N}\left( {\lambda_{1} + \lambda_{2} + \ldots\mspace{11mu} + \lambda_{N}} \right)} \right\rbrack} = {N.}$

Accordingly, the ratio of the expected gain of the weakest spatialchannel to the overall expected gain is

$\frac{E\left\lbrack \lambda_{N} \right\rbrack}{E\left\lbrack {\frac{1}{N}\left( {\lambda_{1} + \lambda_{2} + \ldots\mspace{11mu} + \lambda_{N}} \right)} \right\rbrack} = {\frac{1}{N^{2}}.}$

As shown above, the gain of the weakest spatial channel is 1/N² of theoverall expected gain. For example, the gain of the weakest spatialchannel is 9.5 dB below the overall expected gain in a 3×3 system and is12 dB below the overall expected gain in a 4×4 system. In the variousembodiments of the present invention, this weakest spatial channel isalways punctured for N>2, and the size of the feedback matrix becomesN(N−1) instead of N². This reduces not only the feedback bandwidth butalso the computational complexity because the receiver now needs tocompute N−1 beamforming vectors instead of N beamforming vectors andutilizes N spatial channels. In addition to reducing the feedbackbandwidth, the performance of the communications link as measured byvarious parameters may increase as a result of always puncturing onespatial channel.

FIG. 3 shows simulation results comparing the performance of oneembodiment of the present invention, as well as the performance of a MLsystem and a system that feeds back all N beamforming vectors. Theperformance measure shown in FIG. 3 plots the packet error rate vs.E_(b)/N₀ of a 4×4 48-tone OFDM system using a 64-state convolutionalcode, space-time interleaver, and 64-QAM with hard-decisiondemodulation. As can be seen in FIG. 3, in a 4×4 system, when thereceiver drops the weakest spatial channel and only sends threebeam-forming vectors, the system performance approaches the ML openloopreceiver and is much better than that of sending all beamformingvectors.

FIG. 4 shows a transmitter with digital beamforming. Transmitter 400 maybe included in a station such as station 102 or station 104 (FIG. 1).Transmitter 400 includes data sources 402, digital beamforming block410, radio frequency (RF) blocks 422, 424, 426, and 428, and antennas432, 434, 436, and 438. Digital beamforming block 410 receives threedata signals from data sources 402 and forms signals to drive fourantennas. In operation, digital beamforming block 410 receives channelstate information (CSI) on node 412. In some embodiments, the channelstate information is in the form of beamforming vectors received fromanother station. In embodiments represented by FIG. 4, digitalbeamforming block 410 receives three beamforming vectors, each of lengthfour. This corresponds to a N×N−1 feedback matrix with N=4.

Transmitter 400 always punctures one spatial channel. In the exampleembodiments represented by FIG. 4, N=4, one spatial channel is alwayspunctured, and three spatial channels are always used. Because threespatial channels are always used, data sources 402 only includes threebaseband data circuits to source three separate data streams. This iscontrast to a transmitter that includes four baseband data circuits tosource four separate data streams, even though one may be punctured.

Radio frequency blocks 422, 424, 426, and 428 may include circuitry tomodulate signals, frequency convert signals, amplify signals, or thelike. For example, RF blocks 422, 424, 426, and 428 may include circuitssuch as mixers, amplifiers, filters, or the like. The present inventionis not limited by the contents or function of RF blocks 422, 424, 426,and 428.

Transmitter 400 may include many functional blocks that are omitted fromFIG. 4 for ease of illustration. For example, transmitter 400 mayinclude a scrambler, a forward error correction (FEC) encoder,interleaver, an M-ary quadrature amplitude modulation (QAM) mapper andother functional blocks.

The various items shown in FIG. 4 may be implemented in many differentways. For example, in some embodiments, portions of transmitter 400 areimplemented in dedicated hardware, and portions are implemented insoftware. In other embodiments, all of transmitter 400 is implemented inhardware. The present invention is not limited in this respect.

FIG. 5 shows dimensions of a channel state information matrix. Matrix500 represents a channel state information matrix that may betransmitted back to a transmitter from a receiver. In some embodiments,matrix 500 corresponds to a beamforming matrix V, described above,having dimensions N×N−1. This corresponds to an N×N MIMO system thatalways punctures one spatial channel. In embodiments in which N=4, abeamforming matrix having the same dimensions as matrix 500 may be inputto digital beamforming block 410 at node 412 (FIG. 4).

FIG. 6 shows a transmitter with analog beamforming. Transmitter 600 maybe included in a station such as station 102 or station 104 (FIG. 1).Transmitter 600 includes data sources 610, RF blocks 612, 622, and 624,analog beamforming block 630, and antennas 642, 644, 646, and 648.Analog beamforming block 630 receives three RF signals from RF blocks612, 622, and 624 and forms signals to drive four antennas. Inoperation, analog beamforming block 630 receives channel stateinformation (CSI) on node 632. In some embodiments, the channel stateinformation is in the form of beamforming vectors received from anotherstation. In embodiments represented by FIG. 6, analog beamforming block630 receives three beamforming vectors, each of length four. Thiscorresponds to a N×N−1 feedback matrix such as matrix 500 (FIG. 5) withN=4.

Transmitter 600 always punctures one spatial channel. In the exampleembodiments represented by FIG. 6, N=4, one spatial channel is alwayspunctured, and three spatial channels are always used. Because threespatial channels are always used, data sources 610 only includes threebaseband data circuits to source three separate data streams. Further,because three spatial channels are always used, transmitter 600 only hasthree RF blocks 612, 622, and 624. This is contrast to a transmitterthat includes four baseband data circuits and four RF blocks to sourcefour separate data streams, even though one may be punctured.

Radio frequency blocks 612, 622, and 624 may include circuitry tomodulate signals, frequency convert signals, amplify signals, or thelike. For example, RF blocks 612, 622, and 624 may include circuits suchas mixers, amplifiers, filters, or the like. The present invention isnot limited by the contents or function of RF blocks 612, 622, and 624.

Transmitter 600 may include many functional blocks that are omitted fromFIG. 6 for ease of illustration. For example, transmitter 600 mayinclude a scrambler, a forward error correction (FEC) encoder,interleaver, an M-ary quadrature amplitude modulation (QAM) mapper andother functional blocks.

FIG. 7 shows a system diagram in accordance with various embodiments ofthe present invention. Electronic system 700 includes antennas 710,physical layer (PHY) 730, media access control (MAC) layer 740, Ethernetinterface 750, processor 760, and memory 770. In some embodiments,electronic system 700 may be a station capable of puncturing one spatialchannel. For example, electronic system 700 may be utilized in awireless network as station 102 or station 104 (FIG. 1). Also forexample, electronic system 700 may be a transmitter such as transmittersuch as transmitter 400 (FIG. 4) or 600 (FIG. 6) capable of beamforming,or may be a receiver capable of performing channel estimation anddetermining a weakest spatial channel to be punctured.

In some embodiments, electronic system 700 may represent a system thatincludes an access point or mobile station as well as other circuits.For example, in some embodiments, electronic system 700 may be acomputer, such as a personal computer, a workstation, or the like, thatincludes an access point or mobile station as a peripheral or as anintegrated unit. Further, electronic system 700 may include a series ofaccess points that are coupled together in a network.

In operation, system 700 sends and receives signals using antennas 710,and the signals are processed by the various elements shown in FIG. 7.Antennas 710 may be an antenna array or any type of antenna structurethat supports MIMO processing. System 700 may operate in partialcompliance with, or in complete compliance with, a wireless networkstandard such as an 802.11 standard.

Physical layer (PHY) 730 is coupled to antennas 710 to interact with awireless network. PHY 730 may include circuitry to support thetransmission and reception of radio frequency (RF) signals. For example,in some embodiments, PHY 730 includes an RF receiver to receive signalsand perform “front end” processing such as low noise amplification(LNA), filtering, frequency conversion or the like. Further, in someembodiments, PHY 730 includes transform mechanisms and beamformingcircuitry to support MIMO signal processing. Also for example, in someembodiments, PHY 730 includes circuits to support frequencyup-conversion, and an RF transmitter.

Media access control (MAC) layer 740 may be any suitable media accesscontrol layer implementation. For example, MAC 740 may be implemented insoftware, or hardware or any combination thereof. In some embodiments, aportion of MAC 740 may be implemented in hardware, and a portion may beimplemented in software that is executed by processor 760. Further, MAC740 may include a processor separate from processor 760.

In operation, processor 760 reads instructions and data from memory 770and performs actions in response thereto. For example, processor 760 mayaccess instructions from memory 770 and perform method embodiments ofthe present invention, such as method 200 (FIG. 2) or methods describedwith reference to other figures. Processor 760 represents any type ofprocessor, including but not limited to, a microprocessor, a digitalsignal processor, a microcontroller, or the like.

Memory 770 represents an article that includes a machine readablemedium. For example, memory 770 represents a random access memory (RAM),dynamic random access memory (DRAM), static random access memory (SRAM),read only memory (ROM), flash memory, or any other type of article thatincludes a medium readable by processor 760. Memory 770 may storeinstructions for performing the execution of the various methodembodiments of the present invention.

Although the various elements of system 700 are shown separate in FIG.7, embodiments exist that combine the circuitry of processor 760, memory770, Ethernet interface 750, and MAC 740 in a single integrated circuit.For example, memory 770 may be an internal memory within processor 760or may be a microprogram control store within processor 760. In someembodiments, the various elements of system 700 may be separatelypackaged and mounted on a common circuit board. In other embodiments,the various elements are separate integrated circuit dice packagedtogether, such as in a multi-chip module, and in still furtherembodiments, various elements are on the same integrated circuit die.

Ethernet interface 750 may provide communications between electronicsystem 700 and other systems. For example, in some embodiments,electronic system 700 may be an access point that utilizes Ethernetinterface 750 to communicate with a wired network or to communicate withother access points. Some embodiments of the present invention do notinclude Ethernet interface 750. For example, in some embodiments,electronic system 700 may be a network interface card (NIC) thatcommunicates with a computer or network using a bus or other type ofport.

Although the present invention has been described in conjunction withcertain embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention as those skilled in the art readily understand.Such modifications and variations are considered to be within the scopeof the invention and the appended claims.

1. A method comprising: receiving a training sequence from atransmitter; estimating N spatial channels in amultiple-input-multiple-output (MIMO) system, wherein N is equal to anumber of receiving antennas; performing singular value decomposition todetermine an N×N transmit beamforming matrix; removing one transmitbeamforming vector from the N×N transmit beamforming matrix to yield N−1transmit beamforming vectors, wherein the one transmit beamformingvector removed corresponds to a weakest of the N spatial channels; andtransmitting the N−1 transmit beamforming vectors to the transmitter. 2.The method of claim 1 wherein N is equal to four.
 3. The method of claim1 wherein N is equal to three.
 4. A method comprising always puncturingone spatial channel in an N.times.N multiple-input-multiple-output(MIMO) wireless system to yield N−1 spatial channels, where N is equalto a number of receiving antennas and is greater than one, by alwaysfeeding back only N−1 transmit beamforming vectors from a receiver to atransmitter.
 5. The method of claim 4 wherein N is equal to four.
 6. Themethod of claim 4 wherein N is equal to three.
 7. A computer-readablemedium encoded with instructions that when executed by a computer causethe computer to perform: receiving a training sequence from atransmitter; estimating N spatial channels in amultiple-input-multiple-output (MIMO) system, wherein N is equal to anumber of receiving antennas; performing singular value decomposition todetermine an N×N transmit beamforming matrix; removing one transmitbeamforming vector from the N×N transmit beamforming matrix to yield N−1transmit beamforming vectors, wherein the one transmit beamformingvector removed corresponds to a weakest of the N spatial channels; andtransmitting the N−1 transmit beamforming vectors to the transmitter. 8.The computer-readable medium of claim 7 wherein the channel stateinformation includes a beamforming matrix to cause the transmitter toutilize N−1 spatial channels.
 9. The computer-readable medium of claim 7wherein the channel state information describes spatial channels in anorthogonal frequency division multiplexing (OFDM)multiple-input-multiple-output (MIMO) system.
 10. A wirelesscommunications device having N antennas, the wireless communicationsdevice having a combination of hardware and software components todetermine and a weakest of N spatial channels and to always puncture theweakest of N spatial channels, wherein the wireless communicationsdevice includes a combination of hardware and software to transmit N−1beamforming vectors to a transmitter for use in antenna beamforming intoN−1 spatial channels, where N is greater than one.
 11. The wirelesscommunications device of claim 10 wherein the wireless communicationsdevice includes N−1 baseband data circuits to source data to abeamforming network.
 12. The wireless communications device of claim 10wherein N is equal to four, and three spatial channels are always used.13. The wireless communications device of claim 10 wherein N is equal tothree, and two spatial channels are always used.